Lapping is a well established process for finishing the tooth surfaces of gears such as bevel and hypoid gears. It is a process that provides an economical alternative to other hard finishing processes for bevel gears and it has been used in most gear manufacturing applications.
In the lapping process, a pinion and ring gear are mounted, via appropriate workholding equipment, to respective spindles in a lapping machine that has the same basic design as a testing machine. In most instances of rolling of the gearset, the pinion is the driving member and the ring gear is braked. The gears are rolled in mesh and lapping compound, which can be a mixture of oil (or water) and silicon carbide or similar abrasive, is poured into the meshing zone. One example of a lapping or testing machine can be found in U.S. Pat. No. 6,120,355 to Stadtfeld et al.
Most lapping and testing machines have three degrees of freedom available for realizing relative motion between a ring gear and pinion. The first freedom being relative movement G in the direction of the ring rear axis Z, the second freedom being relative movement H in direction of the pinion axis X, and the third degree of freedom being distance V between the ring gear and pinion axes in the direction of axis Y which is usually arranged perpendicular to the X and Z axes. The distance V is also known as the “hypoid offset.”
In lapping or testing processes, relative movement in the V and H directions effect positional changes in the contact pattern of the members of the gearset, in effect modifying the contact pattern. Lapping involves rotating the gear members in mesh with contact at a desired position on the tooth surfaces. Thus, the members are located at particular V and H positions along with a particular position G to effect the desired backlash.
The position of axes of machines used for lapping and testing bevel gearsets has often been gaged manually through the use of job-specific set-up gages sometimes known as cone gages. These gages are typically employed once or more per work shift to insure that the automatic machine axes are properly positioning the gear and pinion workpieces relative to one another. They can detect and compensate for many possible sources of error such as machine thermal growth, CNC scale errors, tooling dimension uncertainty, etc.
These cone gages are used in pairs, one member being clamped (chucked) in place of the gear into the gear spindle workholding equipment, and the other chucked in place of the pinion into the pinion spindle workholding equipment. The two members of a cone gage pair, therefore, have several mechanical features in common with the gear and pinion, respectively, sufficient to allow them to be held by the same workholding equipment.
When the machine axes are positioned to hold a gearset in the nominal running position designed for that gearset, the machine is in the “job position”. A cone gage pair indicate a gage position. If the gage position is identical to the job position, then the cone gages can be referred to as absolute cone gages. The job position is defined in three dimensions which can be given as the “pinion cone,” “gear cone” and “offset” (or typically also by the X, Y, and Z axes of the machine tool). In other words, a machine whose axes are positioned to hold a gearset in its nominal job home position will hold the associated cone gage members in a “zero” or “gage” position indicated by the gages themselves.
Some existing cone gages indicate the gage position of all three axes mechanically by having a surface of one gage member just touching a surface of the other. Others work with one or more high-precision dial indicators built into the cone-gages such that when the indicator reads zero, the axis in question is in the gage position. Still others work with a combination of both methods. For maximum accuracy, it has been historically preferable that for absolute cone gaging all three axes be brought into the job position simultaneously and that the cone gages indicate the gage position in all three dimensions simultaneously. In general, these cone-gage methods result in axes positioning accuracies of 0.0002 inch or less.
It can be noted that cone gages or the equivalent have been designed for angular bevel sets (gearset angles other than 90 degrees) and cylindrical gearsets as well.
Traditionally, absolute cone gages are operated manually by the machine operator or technician, specially trained for the operation. The operator clamps the gages into the workholding fixtures and then brings them carefully into engagement using machine manual mode commands to jog the axes into the job position as indicated by the gages. This frequent operation interrupts production and can be slow and tedious. Results can even be subjective, as when an operator judges the light metal-to-metal contact condition required by most of these gages.